The functionalisation of thenyl carbamates.
The carbamate functionality has always been associated with a major class of biocides because of its ability to function as an inhibitor of the enzyme acetylcholinetransferase. However, carbamates are not limited to pesticidal applications, they have also shown potential as intermediates in organic synthesis. Research has shown that amongst its other properties, the carbamate group has the ability to migrate, function as a leaving group and participate in rearrangement reactions. As part of ongoing research at this department on the synthetic utility of the carbamate group, this project has been primarily concerned with the chemistry of carbamates in conjunction with thiophene, an aromatic heterocycle. The thiophenes also represent an important class of organic compounds. They are found in natural products, biologically and pharmacologically active systems (both naturally occurring and synthetic), synthetic precursors and more recently, organic conductors and electro-optical devices. In exploring the chemistry of thiophene carbamates, the unique nature of the thiophene ring has been shown to affect the synthesis and reactions of these compounds; the often peculiar character of thiophene imparts properties which make these carbamates remarkable and distinct from carbamates of other aromatic and conjugated systems such as benzene and conjugated polyenes. With a wide range of potential applications, the purpose of this project has been to study the synthesis and reactions of thiophene carbamates, in particular, the reaction of deprotonated thenyl carbamates with electrophiles, using the electrophile as a means of studying the charge delocalisation in such systems. A series of thenyl carbamates has been synthesised from thenyl alcohols and their reactions with electrophiles have been studied. The electrophilic substitution reactions illustrate the possibilities for the functionalisation of these compounds, particularly remote functionalisation via anionic charge migration; the charge has been found to migrate across five carbon atoms, a phenomenon not observed in this department's studies of other carbamate systems. The substitution products which form in these reactions depend on the nature of both the carbamate and the electrophile. In addition, three rearrangements (carbamate to amine, substituted carbamate to alkene, and substituted carbamate to a-hydroxyamide) have been observed which may find application in organic synthesis. The potential uses for these carbamates range from biologically active applications as carbamates or as derivatives (amines or alkenes via rearrangement or alcohols via deprotection); many contain a chiral centre and thus may be used in enantioselective or diastereoselective processes in the synthesis of other products, as well as the industrial applications in the fields of non-linear optics and conducting polymers.